This invention relates to electronic drive circuits for an active matrix device (for example, a liquid crystal display), comprising circuit blocks each having redundancy in the form of parallel circuit paths, and further relates to a method of self-testing and programming such circuits. The invention relates particularly, but not exclusively, to the provision of redundancy in a shift register for such an active matrix device, and the circuit may be composed of thin-film circuit elements formed on an insulating substrate of the active matrix device.
The integration of a drive circuit for an active matrix device onto the device substrate offers a number of advantages over the more conventional approach of providing external connections from the active matrix device to separate drive circuits in the form of conventional silicon integrated circuit chips. These advantages include a more compact device, a reduced complexity of electrical interconnection and a possibility of reduced manufacturing cost. If the drive circuit is fabricated using thin-film circuit technology compatible with that used to make the active matrix device, then the drive circuit could in principle be produced on the device substrate at little or no additional cost. However, this assumes that the drive circuit integration does not significantly affect the yield of working devices. A fault in the drive circuits (such as a short or open circuit) could result in otherwise good devices having to be rejected. This problem is particularly acute for a shift register which forms one of the main elements of conventional row or column drive circuits.
With such a conventional row drive circuit, the active matrix is scanned by feeding a pulse into the input of the shift register. This input pulse must pass correctly through all the stages of the shift register in order for the drive circuit to operate correctly. A fault within one register stage will result in all of the following stages producing incorrect shift outputs. It is therefore important that the register should be free of faults. This problem is considered in U.S. Pat. No. 5,063,378, the whole contents of which are hereby incorporated herein as reference material.
The electronic drive circuit proposed in U.S. Pat. No. 5,063,378 is divided into a series of circuit blocks, each block having redundancy in the form of parallel circuit paths which provide alternative routing through the block. The circuit also comprises a switch for selecting from each block which of the parallel circuit paths is to be used to provide an output from that block. The blocks in the circuits of U.S. Pat. No. 5,063,378 comprise only a single stage of the shift register, and the switch is a combiner circuit coupled between the blocks to control which serial output of the parallel circuit paths of each stage of the register is transmitted to the next stage of the register. That U.S. patent does not describe how the combiner circuit or driver circuit may prevent the scanning signal from both parallel registers appearing on the row conductor. The parallel paths may be on one side of the active matrix or, for example, on opposite sides of the matrix. The active matrix device of U.S. Pat. No. 5,063,378 is a liquid crystal display (LCD).
After manufacture of the LCD array with all its circuitry, the drive circuit of U.S. Pat. No. 5,063,378 is tested by sequentially activating each successive row of the active matrix using one (first) of the shift register paths. If a fault is detected in a stage of this first shift register path, the combiner circuit coupled to the output of the defective stage has then to be reconfigured to route the scanning signal from the corresponding stage of the parallel (second) shift register path to the next stage of the first shift register path. This is carried out as a final stage in the manufacture of the device. In most cases the combiner circuit is reconfigured by breaking a fusable link with a laser beam. However, accurate positioning of the laser beam on the link is required to reconfigure the combiner circuit in this way, otherwise adjacent parts of the circuit may be damaged. It is also suggested in U.S. Pat. No. 5,063,378 that the use of a laser can be avoided by applying instead a special potential to one or more inputs of the combiner circuit so as to condition the combiner circuit to reroute the scanning signal.
Although the provision of shift register redundancy in this manner provides a considerable improvement in the yield of working displays, further significant improvements can be made by adopting in accordance with the present invention a different approach to the testing and control of the redundancy.